Ocean with mirror cube
I tried it in Eevee and it was fun :).
I used this free HDRI to light the scene.

Cube


Metalness to 1 for highest reflectivity (just like real mirrors)
Musgrave and Noise texture for those tiny black dots
Layer Weight > Facing to control lighter/darker side.
Dark edges. Add a Bevel modifier and set Material: 1


Then add a second material slot to your object with some dark material



Ocean


Wave Modifier on a subdivided plane for large waves
Voronoi texture as a Bump map for those tiny ripples








The HDRI isn't packed, just download and put it next to the Blendfile :).